1. Field of the Invention
The present invention relates to an optical information recording medium configured to be capable of recording and reproducing record data by irradiating a laser beam to a recording layer formed on a substrate.
2. Description of the Related Art
As an optical information recording medium of this kind, the present assignee has proposed a single-sided write-once optical information recording medium in Japanese Patent Application No. 2002-105994. The optical information recording medium is comprised of a substrate formed of a polycarbonate or the like by an injection molding method, and a reflective layer, a recording layer, a protective layer, and a covering layer, deposited on the substrate. The reflective layer reflects a laser beam irradiated from the covering layer side when record data is recorded and reproduced, and is in the form of a thin film made of a metal, such as Au, Ag, or Al, or a mixture of selected ones of these metals. The recording layer is provided in a manner sandwiched between a first protective film and a second protective film constituting the protective layer. The recording layer is formed by sequentially depositing a second sub-recording film and a first sub-recording film on the second protective film in the mentioned order. In this case, the first sub-recording film is formed of a highly reflective metal, such as Al, Cu, Ag, or Au, and the second sub-recording film is formed of a reactive metal, such as Sn, Te, Sb, Ge, Si, or C, or a semimetal. On the other hand, the first and second protective films (dielectric layers) of the protective layer prevent oxidation of the sub-recording films during irradiation of the laser beam, and deformations of the substrate and the covering layer, and is formed of a dielectric material, such as ZnS+SiO2. The covering layer (light-transmitting layer) is formed of e.g. a light-transmitting resin applied by spin-coating, and provides an optical path of the laser beam while preventing the layers on the substrate from being scratched.
In recent years, optical information recording media are desired to have the capability of recording and reproducing a large amount of record data at a high speed (in a short time period). Accordingly, the optical information recording medium of the above-mentioned kind is required to enhance recording density of record data, and to meet the requirement, the diameter of a beam spot of a laser beam used for recording or reproduction of record data tends to be reduced. More specifically, record data are recorded and reproduced using a pickup which is equipped with an objective lens having a numerical aperture (NA) of not less than 0.7 (e.g. a numerical aperture (NA) of approximately 0.85), and is capable of emitting a laser beam having a wavelength (λ) of not more than 450 nm (e.g. a wavelength (λ) of approximately 405 nm). However, as the numerical aperture (NA) is larger, the allowable angle range (i.e. tilt margin) of the optical axis of the laser beam with respect to the optical information recording medium becomes smaller. Therefore, in the case of the construction of the optical information recording medium, such as a conventional general type, in which the laser beam emitted from the pickup is irradiated through the substrate to the recording medium, the substrate has a rather large thickness of e.g. approximately 1 mm, and therefore it is difficult to obtain a desired tilt margin. To overcome this problem, the optical information recording medium proposed by the present assignee employs the construction which allows the laser beam to be irradiated through the covering layer having a thickness of approximately 100 μm, which is formed in a manner covering the recording layer, to the recording layer, whereby a tilt margin is obtained which is large enough to stably record and reproduce record data. In this case, to suppress coma of the laser beam as well, it is preferable to reduce the thickness of the covering layer (e.g. to a value of e.g. 100 μm).
To record data on the optical information recording medium (forming pits according to contents of record data), a laser beam adjusted to a recording power is irradiated to the recording layer. The irradiation of the laser beam causes the two sub-recording films to be mixed with each other (atomic arrangement thereof to be changed) to form recorded portions (pits). On the other hand, to reproduce the record data recorded on the optical information recording medium (determine whether or not each pit exists), a laser beam adjusted to a reproducing power is irradiated to the recording layer. The recorded portions and the unrecorded portions are different in optical constant therebetween, so that the irradiation of the laser beam to these portions reveals different values of reflectance thereof. Therefore, by detecting the difference, it is possible to determine the existence of each pit in the recording layer (whether or not the pit exists in the recording layer) to thereby reproduce the record data.
The present inventors have studied the above-described optical information recording medium, and found the following points to be improved: the optical information recording medium employs the construction which allows the laser beam to be irradiated through the covering layer formed in a manner covering the recording layer, to the recording layer. In this case, when the optical information recording medium is produced, there are the reflective layer, the second protective layer, the second sub-recording film, the first sub-recording film, the first protective layer, and the covering layer, sequentially deposited on the substrate in the mentioned order. In this case, as shown in FIG. 5, the reflective layer (3) formed on the substrate (2) which is formed such that the surface of each guide track (upper surface of land and bottom surface of groove: upper surface of the substrate (2) as viewed in FIG. 5) is flat has a slightly rough surface compared with the surface of the substrate (2). It should be noted that in FIG. 5, the guide track (groove and land) is omitted from illustration for ease of understanding the multilayer structure. Further, the second protective layer (5b) formed on the slightly rough surface of the reflective layer (3) has a rougher surface than that of the reflective layer (3). Therefore, after the layers i.e. the reflective layer (3) to the covering layer (6), are sequentially deposited, the surface of each layer becomes rougher as the layer is more distant from the substrate (2) (i.e. as closer to incidence plane of the laser beam). FIG. 5 shows the progressively roughened conditions of the respective surfaces of the layers in an exaggerated manner for purposes of illustration.
In the above case, when the upper surface of the second protective layer (5b) (interface between the second protective film (5b) and the second sub-recording film (4b)), the upper surface of the second sub-recording film (4b) (interface between the second sub-recording film (4b) and the first sub-recording film (4a)), and the upper surface of the first sub-recording film (4a) (interface between the first sub-recording film (4a) and the first protective film (5a)) are significantly rough, the level of noise contained in a reproduction signal (noise level) is high, so that the C/N ratio is lowered. Particularly, in the case of the optical information recording medium (1) of a type to which a laser beam having a short wavelength is emitted from an objective lens having a large numerical aperture to record and reproduce record data, the laser beam irradiated to the sub-recording films (4a and 4b) forms a beam spot having a very small diameter, so that the surface smoothness of the second sub-recording film (4b) has large influence on the noise level of the reproduction signal and the value of the C/N ratio. Therefore, depending on the reproduction speed, it can be difficult to accurately read record data recorded on the optical information recording medium (1), since the surface of each layer thereof tends to be formed to be rougher as the layer is closer to the incidence plane of the laser beam. To overcome this problem, it is desirable to reduce the noise level and improve the C/N ratio. Further, this kind of optical information recording medium is desired to be capable of storing record data for a long time period such that the record data can be normally reproduced during the time period, and therefore, it is desirable to realize the optical information recording medium improved in this respect.